1. Field of the Invention
The present invention relates to an image capture device which can be used for a digital camera, and is provided with an automatic-focusing function and to a method for automatic-focusing control.
2. Description of the Related Art
Conventionally, a digital still camera (hereinafter, called a digital camera) provided with an automatic-focusing function according to an automatic focusing (AF) method of a contrast detection type has been known.
The digital camera provided with an automatic-focusing function extracts a high frequency component by using a high pass filter after necessary preprocessing of image data for a subject image formed with a lens unit when a shutter button is half pressed. Then, image formation can be realized at the best focus position by driving the lens unit and adjusting a focus position in such a way that the high frequency component of the whole data is a maximum value.
According to a method for detecting a focus position in such a way that the high frequency component is the maximum value, it is determined, by the above-described processing of the image data at a necessary interval while the focus position is changed from infinite distance to this side by driving the lens unit, whether or not the high frequency component is the maximum value. Then, the image data of the subject image is recorded, wherein the subject image has been captured at the best focus position by finally driving the lens unit to a focus position at which the high frequency component is the maximum value, and by full-pressing of the shutter button under a state that the high frequency component is the maximum value.
Even when the shutter button is fully pressed at one stroke, a half-pressed state is always passed to start focusing adjustment, the subject image is captured at the best focus position, and the image data for the subject image is recorded.
However, a conventional digital camera provided with an automatic-focusing function according to an AF method of a contrast detection type requires image data processing and driving control of a lens unit for determining an appropriate lens position at which a subject is in focus, as described above. Accordingly, there have been problems: a problem that the shutter is not released at the right moment because there is necessary time between time at which a camera user catches a subject in a finder and time at which the subject is in focus and a shutter button is fully pressed for image forming; and a problem that a subject image with some lag between a full-pressing point and a point at which an image captured by the camera user might be obtained is recorded even when the shutter button is fully pressed at one stroke because the recorded image is an image which has been formed when the image is in the best focus.
In order to solve the above-described problems, there has been disclosed an image capture device (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-085697 (Patent Document 1)) for capturing a subject image in a state in which the shutter is released at the right moment and the time lag is eliminated, thereby recording the subject image. According to the above image capture device, it is discriminated, when an image is captured, whether an interrupt processing flag is ON or OFF. When a shutter button is “half pressed”, the interrupt processing flag is set to be ON, and it is determined whether or not there is a “fully pressed” signal from the shutter button. When the shutter button is “fully pressed” at one stroke, the determination is assumed to be YES. Accordingly, a charge coupled device (CCD) outputs image data for exposure, and the image data for exposure is separated to a luminance signal and a color difference signal in a color processor. A central processing unit (CPU) stores the separated luminance signal and color difference signal into a RAM. Consequently, image data, which is obtained when the shutter button is fully pressed before focusing is completed, is configured to be stored in the RAM.
Moreover, there is an advantage that time for detecting a focused position can be reduced in driving control of a lens unit when the driving speed of the focus lens is increased, but there is a disadvantage that it is difficult to accurately stop the focus lens at a focused position. There has been a technology (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-287012 (Patent Document 2)) which brings the above-described advantage and the above-described disadvantage into harmony with each other, based on the above-described viewpoint, by changing the driving speed of the focus lens according to situations and by detecting the focused position.
According to the patent document 2, when a release button (shutter key) is half pressed, low-speed driving of the focus lens is started, and subsequently, a difference between the current position of the focus lens and a focused position is measured. If the measured value is smaller than a predetermined value, the focus lens is driven at a low speed with no change in the speed. If the measured value is smaller than the predetermined value, time for detecting the focused position is shortened by driving the focus lens at a high speed.